how do I add two AC power signal ?

[retraeh]

I see sb asked before, just don't seems right. anyone can explain simply how to add two 110V AC together? one example will be what happens if you connect two solar inverters output together? do you still get 110V AC?

 

[WimRFP]

Hello,

No this will not work.

When you turn on both units, the phase relation is unknown. Also the frequencies will not be equal, so you have changing phase relation between the signal. Worst case they can add (in phase) so you get twice the output voltage. Worst case they can be in opposite phase and you get nothing.

Putting them into parallel circuit also doesn't work. When they are in opposite phase, both units will short-circuit each other.

To enable addition of sources, you need to synchronize them (in both frequency and phase).

 

[retraeh]

assume both interters are in synch. output phases are in synch. what do I get 110V or 220VAC?

 

[WimRFP]

Hello,

When you put them in parallel you get 110VAC, in series connection you get 220VAC. Whether current sharing is OK, depends on the output impedance of the inverters and voltage tolerance. There is a ver large but....

If they don't have isolation between AC out and DC in, put them in series and feed them from one DC source, you will get a short circuit. So make sure what you are doing.

 

[retraeh]

thanks for your input.

I think creating short circuit is not that easy, usually you have transformer isolation between AC output and DC input. solar panel DC out is usually 24V or 48V, inverter output usually 120VAC, so you will have to use 1:5 or 1:2.5 transformer. correct me if I am wrong.

one thing you might want to check out is Enphase microinverter, they connect inverters output in paralell, 16 inverters can be connected according to their website.
sb mentioned their products on this board before, I just can not figure out how they do it. all these inverters use the same clock to switch output transistors?

Added after 3 minutes:

check out link below from their video
http://www.enphaseenergy.com/products/moreinfo/howitworks.cfm

I don't believe microinverter is that good at all. but they seem to convinced VC

 

[FvM]

The said "microinverter" is intended for grid connected operation. It's synchronizing itself to the grid voltage and frequency. As any grid connected inverter, it can be operated in parallel. There's a limit by the risk of getting "islanding" effects with a too high grid impedance, depending on the inverter design.

A series connection isn't possible, however.

Generally, the promise of "microinverter" and similar designs is to achieve MPPT for individual solar panels, independant of efficiency differences and shading of individual modules. Basiaclly, it's a great idea.

 

[hearter]

enphase microinverters are not for grid connection if you look at their video. that is why I am wondering how do they synch phase of difference inverters?

also anyone know are they using transformer at output or transformerless?

 

[FvM]

enphase microinverters are not for grid connection if you look at their video.

It's exactly the other way around. The microinverter FAQ clarifies:

Can Enphase Microinverters work in off-grid environments?
Enphase Microinverters have only been tested to work in grid-tied installations.

This statement seems understandable so far. In off grid installation, the inverter misses a frequency and voltage reference. For security reasons, grid-tied inverters are required to switch off in case of grid failure.

To achieve 230 VAC output with a single solar module on input, they have to use a high frequency transformer, of course.

 

[hearter]

it is confusing from their video, make sense they need frequency from grid. what do you mean voltage reference?

FvM , are you saying enphase use transformer? I thought they are transformerless according to one of their patent. they just used DC-DC boost followed by SCR controller and H-bridge before it goes to grid.
anythoughts?

 

[FvM]

what do you mean voltage reference?

I can only answer generally, because I don't know the product details. An ideal grid-tied inverter can be imagined as a sine current source, phase locked to the grid. Real inverters are rather voltage sources with a finite (mostly inductive) impedance, they can be controlled much better when knowing the instantaneous grid voltage and adding it to the current control loop output (voltage feed forward).

they just used DC-DC boost followed by SCR controller

May be, I don't see much difference between a transformer DC/DC converter and a boost converter, that needs an inductor of similar size. The boost converter must be expected to have a smaller efficiency due to higher switching losses at e.g. 1:10 voltage ratio. In my opinion, it's just a design detail.

Do you know the respective patent application number?

 

[hearter]

check out this pat20090244947, what do you think now? they seem to use no transformer.

 

[FvM]

The application doesn't say anything about the nature of the DC/DC converter, if it's transformer based or not. Simply, it's beyond the patent's scope.

By the way, you shouldn't conclude from the existence of a patent, that the described technique is applied in a product. The patent isn't referenced in the microinverter datasheet, where do you see it?

Technically, I doubt, that SCRs can compete with MOSFET for a medium power inverter. Because efficiency is a keyword for solar inverter marketing, and it's counted by 0.1%.

 

[hearter]

FvM,
I agree, SCR has no advantage over MOSFET. MOSFETs can be integrated as well as advantage. But this patent seems used in their microinverter as a transformerless topology.

question for you, have you seen people use several H-Bridges to connect together at output? see attached file. I think this is same as connecting inverters output in parallel, instead it is done at H Bridge node. do you see any current cross flowing?

Hearter

 

[FvM]

Connecting H-bridges in parallel without separate output inductors isn't a good idea in my opinion. I can't see a reasonable purpose for it. It can only work, if both bridges have identical bus voltage and exactly identical control waveforms.

If "transformerless" means not to use a bulky mains frequency transformer, all modern inverters are actually transformerless. But the DC/DC converter most likely uses a (high frequency) transformer, and I'm not aware, that the "microinverter" is designed different.

 

[hearter]

why DC DC converter need transformer? it only need inductor I think.

 

[Fidelis Anyanti]

Hi,

hearter wrote :

why DC DC converter need transformer? it only need inductor I think.

DC to DC converter takes 12v or any other voltage and inverts it to AC at the desired voltage with high frequency transformer ( Chopper transformer ), the AC is then rectified to get DC.

Cheers.